Field of the Invention
The present invention relates to a method of modifying refractory particles for a casting mold, refractory particles for a casting mold obtained by the method, and a method of producing a casting mold. The present invention relates more particularly to a technique of modifying refractory particles used as a molding sand that constitutes a casting mold used for sand mold casting performed by using metals in a molten state, such as aluminum, an aluminum alloy, cast iron, cast steel, copper and a copper alloy. The technique permits an advantageous improvement of the strength of the casting mold produced by using the refractory particles.
Description of Related Art
As the casting mold used for casting a desired molten metal such as aluminum or an aluminum alloy described above, there has been used a sand mold produced by forming a molding sand composition obtained by kneading a molding sand in the form of refractory particles or aggregate with a binder that bonds the refractory particles together. As one type of the sand mold, an organic self-curing mold which is called a furan mold and produced by using a furan resin as the binder is well known.
As the refractory particles (molding sand) giving the casting mold described above, natural sands such as silica sand, zircon sand, olivine sand and chromite sand are widely used. However, the naturally produced sands have undesirable variation in their quality such as physicochemical properties and an inherent problem of exhaustion due to recent reduction of natural resources. Therefore, the use of artificially produced refractory particles has been proposed, and various kinds of artificial aggregate containing mullite or corundum as a major mineral have been practically used.
For example, JP-B-3(1991)-47943 and JP-B-4(1992)-40095 propose a method of producing a spherical molding sand by preparing a slurry having a chemical composition containing 20-70% by weight of Al2O3 and 80-30% by weight of SiO2, and spraying the slurry into a hot air by using a spray drier to obtain spherical particles having a diameter of 0.05-2.0 mm, and then sintering the obtained particles within a rotary kiln. JP-A-2003-251434 discloses a method of producing a molding sand in the form of spherical particles mainly composed of a synthetic mullite and having a chemical composition containing 40-90% by weight of alumina and 60-10% by weight of silica, by blowing an air on a raw material composition melted at a temperature of 1600-2200° C. Further, JP-A-2004-202577 discloses a technique of producing, by a flame fusion method, a spherical molding sand which is mainly composed of Al2O3 and SiO2 contained at a weight ratio Al2O3/SiO2 of 1-15, and which has an average particle diameter of 0.05-1.5 mm and a sphericity of not less than 0.95.
In the case where the artificial aggregate in the form of the artificially produced refractory particles is used to produce the casting mold by using an organic binder, a larger amount of the organic binder is required than in the case where the naturally produced silica sand or the like is used. Therefore, where substantially the same amount of the organic binder is used in production of the casting mold, the use of the artificial aggregate gives rise to an inherent problem of a lower degree of the strength of the casting mold. In this respect, it is noted that the amount of the binder is adequately determined depending on an application of the casting mold. A larger amount of the binder generally results in a higher degree of the strength of the casting mold, so that the casting mold having a desired structure can be obtained by using a larger amount of the binder. However, an excessively large amount of the binder is not desirable since it leads to undesirable increases of a cost and a risk of a gas defect which may be caused in practical use of the casting mold. On the other hand, an excessively small amount of the binder leads to an insufficient degree of the strength of the casting mold, resulting in failure to obtain the casting mold having the desired structure. Under the above-described circumstances, JP-A-2003-251434 and JP-A-2004-202577 described above and JP-A-2011-25310 disclose that the required amount of the binder can be reduced by producing the artificial aggregate at a higher temperature, to densify the particles of the aggregate and to reduce an apparent porosity of the particles.
However, there is a limitation in the reduction of the required amount of the binder achieved by the above-described conventional means for densifying the particles. Therefore, it is desired to establish a novel technique of modifying the artificial aggregate, which technique permits an improvement of the strength of the casting mold without an increase of the required amount of the binder, to further reduce the required amount of the binder.